The present invention generally relates to electric power systems including intelligent electronic devices (IEDs) for protecting, monitoring, controlling, metering and/or automating electric power systems and associated power lines. More specifically, the present invention relates to a system, method and device for preserving current differential protection communication active during the process involved in a breaker bypass or similar operation.
Electric utility systems or power systems are designed to generate, transmit and distribute electrical energy to loads. In order to accomplish this, power systems generally include a variety of power system elements such as electrical generators, electrical motors, power transformers, power transmission lines, buses and capacitors, to name a few. As a result, power systems must also include IEDs and procedures to protect the power system elements from abnormal conditions such as electrical short circuits, overloads, frequency excursions, voltage fluctuations, and the like.
Generally, IEDs are also used for protecting, monitoring, controlling, metering and/or automating electric power systems and associated power lines. For example, certain IEDs and procedures may act to isolate some power system element(s) from the remainder of the power system upon detection of an abnormal condition or a fault in, or related to, the power system element(s). IEDs may include protective devices such as protective relays or otherwise, remote terminal units (RTUs), power line communication devices (PLCs), bay controllers, supervisory control and data acquisition (SCADA) systems, general computer systems, meters, and any other comparable devices used for protecting, monitoring, controlling, metering and/or automating electric power systems and their associated power lines.
In one example, a particular type of IED generally known as a current differential protective relay protects an associated power line by analyzing the current at different terminals of the line. The general implementation of a current differential protective relay is illustrated in FIG. 1A. A current differential protective relay R1 measures the current I1 situated at one bus 102 via current transformer CT1 on an associated power line 108. Another protective relay R2 measures the current I2 situated at another bus 104 via current transformer CT2 on the same power line 108. The current vector quantity I2 (magnitude and angle) measured by protective relay R2 is transmitted to the current differential protective relay R1 via communication link 130.
During operation, the current differential protective relay R1 then calculates a vector sum of the currents [Σ(I1, I2)]. Under no-fault conditions, the resulting vector sum equals about zero amperes. In contrast, the occurrence of a fault or other abnormal condition is detected when the resulting vector sum does not equal about zero amperes. Upon detection of a fault or abnormal condition, the current differential protective relay R1 sends a trip signal or command to an associated circuit breaker 110 to isolate the condition.
In order to fully isolate the fault condition, it is to be noted that the other protective relay R2 is also a current differential protective relay. In this arrangement, the other current differential protective relay R2 may be adapted to concurrently receive the current measurement I1 from current differential protective relay R1 via communication link 130 and calculate a vector sum therefrom.
When protecting, monitoring, controlling, metering and/or automating electric power systems and associated power lines, it is often beneficial to reroute data streams such as communication signals therein in order to calculate maintenance on protective devices or on power system elements associated thereto. For example, a power system element may require maintenance wherein the power system element and its associated protective device must be isolated from its associated power line. In order to maintain power distribution through the power line, power may be rerouted around the element that requires maintenance. In order to maintain protection, control, monitoring etc. of the power line, data streams such as communication signals must also be rerouted.
U.S. Pat. No. 6,639,330 for a “Transfer Relay for Computer Base Equipment” describes a power switching transfer relay to automatically switch an electrical load, such as that drawn by a computer or other sensitive electrical or electronic equipment, from a primary power source to a secondary, or backup, power source upon interruption or loss of the primary source. The transfer relay includes a power relay and two control relays that are arranged to switch the electrical power input from the primary source to the backup source upon failure of the primary power source in the space of less than one cycle, and to actuate an alarm upon loss of the primary power source, loss of the backup power source, or the occurrence of a relay fault.
U.S. Pat. No. 5,347,417 for a “Power Supply Protection System Applied to Optical Subscriber Network” describes a system for protecting a remote power supply for supplying power to an optical subscriber network, via a pair of power supply lines, from a remote power supply apparatus, with the power supply branch apparatuses inserted into the power supply lines in correspondence with each power receiving circuit respectively mounted in subscriber transmission nodes. Each of the power supply branch apparatuses comprises relay contacts inserted into its own power supply branch lines connected between the power supply lines and its own power receiving circuit, and a relay energized by an overcurrent detector or first and second communication units to change over the relay contacts. The relay contacts are opened and closed subscriber by subscriber sequentially to detect a faulty portion, and thereafter, the power is fed again selectively to the subscribers which have not experienced the fault.
U.S. Pat. No. 5,132,867, for a “Method and Apparatus for Transfer Bus Protection of Plural Feeder Lines” describes a microprocessor based tie relay for controlling a tie circuit breaker between a main bus and a transfer bus to which any one of a number of feeder lines may be connected through a disconnect switch when the feeder circuit breaker associated with that feeder line is out of service. Settings for the protection characteristics of each of the feeder relays controlling the feeder circuit breakers are stored in non-volatile memory together with a default protection characteristic suitable for protecting any of the feeder lines. The appropriate protection characteristic for the feeder line connected to the transfer bus is selected for use by the tie relay in controlling the tie circuit breaker. This selection may be made manually by an operator, or preferably automatically by the microprocessor of the tie relay which monitors the states of the feeder circuit breakers and of the disconnect switches and selects the settings associated with the feeder line whose feeder circuit breaker is open and disconnect switch is closed. If the microprocessor does not recognize only one feeder line connected to the transfer bus, the default protection characteristic is selected and an alarm is generated.
U.S. Pat. No. 5,041,737 for a “Programmable Bus-Tie Relay having a Plurality of Selectable Setting Groups” describes a bus-tie relay apparatus which includes a multi-position mechanical switch and a logic circuit responsive to the position of the mechanical switch for producing digital signals on five digital line, wherein a valid digital signal comprises the presence of high conditions on two, and two only, of said digital lines. A sensor senses the condition of the digital lines and retrieves the values of a relay element setting group from memory associated with that digital signal. A plurality of such relay element setting groups are stored in the apparatus, each one of which comprises values corresponding to the characteristics of an in-place relay associated with a particular one power line in a group thereof.
FIG. 1B generally provides an illustration of a traditional system for applying IEDs, such as protective devices, in order to maintain protection, monitoring, controlling, metering and/or automating of an associated power line. It should be clear that while FIG. 1B and other figures (including those illustrating the embodiments of the present invention) show two power lines emanating from a single substation, the methods and systems described herein may be generally extended to more or less than two lines, delivered to one or more substations. In the described systems, local protective relays R1, Rn-1 are associated with respective circuit breakers 110, 111 for primary protection. For primary protection, local protective relays R1, Rn-1 are current differential protective relays similar to those described with respect to FIG. 1A.
In the arrangement of FIG. 1B, local protective relays R1, Rn-1 receive current measurements I2, In from remote protective relays R2, Rn via communication link 130b, 130a in order to preserve current differential protection on power lines 108, 109 as discussed with respect to FIG. 1A. Upon detection of a fault or abnormal condition on power lines 108, 109, the local protective relay R1, Rn-1 associated with that particular power line 108, 109 signals a corresponding circuit breaker 110, 111 to isolate the condition. In order to fully isolate the fault condition, it is to be noted that remote protective relays R2, Rn are also current differential protective relays.
Circuit breakers (e.g., 110 and 111) are high maintenance devices that experience some wear each time they interrupt a fault condition. Accordingly, a substation is typically constructed such that each primary circuit breaker 110, 111 may be taken out of service for maintenance purposes or replacement while leaving its associated power line 108, 109 associated therewith energized. In these instances, prior art arrangements have isolated the primary circuit breaker 110, 111 along with its associated local protective relay R1, Rn-1 in order to provide for secondary protection on the energized power line 108, 109. The local protective relay R1, Rn-1 associated with the primary circuit breaker 110, 111 is commonly referred to as the primary relay.
A method for isolating a primary circuit breaker such as 110 or 111 while providing secondary protection in such instances is commonly referred to as a breaker bypass operation. As shown in FIG. 1B, one traditional arrangement for providing secondary protection in such instances includes having a transfer bus 106 associated with a main or primary bus 102. In this arrangement, to isolate or take primary circuit breakers 110, 111 out of service, all other lines are typically connected to the main bus 102 by proper configuration of switches S2, S5, and other switches as illustrated.
For example, all other power lines are connected to the main bus 102 by closing switch S2, and opening switch S5. During a breaker bypass operation, switch S5, is closed, whereas switches S1, S2, are opened such that power lines 108, 109 are now connected to transfer bus 106. Accordingly, current differential protection of either power line 108, 109 is now maintained through protective relay Rx and circuit breaker 114. The circuit breaker 114 which provides secondary protection is commonly referred to as a transfer breaker, tie breaker, or coupler breaker, whereas its associated relay Rx is commonly referred to as a transfer breaker, tie breaker, or coupler relay. Communication (e.g., communication of current vector quantities as discussed above) between remote relays R2, Rnand transfer relay Rx may be optionally routed through communications switch 200.
Nevertheless, the arrangement of FIG. 1B poses a number of challenges for current differential protection of power lines. For example, current differential protection generally cannot be maintained during the entire bypass process due to the resulting parallel lines that feed a protected power line through both the main and transfer buses during the switching process of a breaker bypass operation. The hypothetical condition of keeping line current differential protection active on the local and remote relays during the switching process, would inaccurately cause these relays to detect a fault or abnormal condition on the power line. This is because the switching process of a bypass operation on the aforementioned bus arrangement, creates a parallel feed path onto the bus, changing the local measured quantity, which causes the vector sums of the currents to be unequal to zero on each relay.
In order to overcome this shortcoming, during a breaker bypass or similar operation, current differential protection is often disconnected and replaced by backup protection such as step-distance. This, however, compromises the quality of the power line protection as step-distance protection is generally known to be slower and less reliable than current differential protection. Most faults associated with a breaker bypass operation generally occur due to human error. For example, operators may inadvertently cause a bus-to-ground fault while they intend to create a parallel current path that will allow for isolation of the circuit breaker. Therefore, during manual modifications to the bus configurations during a bypass operation, the risk of causing a fault is the highest.
Accordingly, it is an object of the invention to provide a system and method for maintaining current differential protection of a power line even during a breaker bypass operation.
This and other desired benefits of the preferred embodiments, including combinations of features thereof, of the invention will become apparent from the following description. It will be understood, however, that a process or arrangement could still appropriate the claimed invention without accomplishing each and every one of these desired benefits, including those gleaned from the following description. The appended claims, not these desired benefits, define the subject matter of the invention. Any and all benefits are derived from the multiple embodiments of the invention, not necessarily the invention in general.